1. Field of the Invention
The invention relates to a Francis runner and to a turbine equipped with such a runner.
2. Description of the Related Art
Francis runners may equip different sorts of hydraulic machines, such as turbines, pumps, or pump turbines. They comprise blades distributed about a central rotating shaft and which define therebetween channels for flow of water. In the case of turbines, the geometry of the blades of these runners is defined so that the flow of the water induces a torque on the runner. The power that a turbine equipped with such a runner can deliver depends on its geometry, particularly on its diameter, in connection with its speed of rotation.
In certain configurations of a Francis hydraulic machine, the diameter of the runner is imposed, particularly in the case of rehabilitation of an installation where the diameter cannot be modified without considerable civil engineering works.
In the case of a conventional Francis turbine, shown partially in plan view and with parts torn away in FIG. 1, the speed V of injection of the water is broken down into a linear speed U of the leading edge B of a turbine blade A and a relative speed W of the jet of water with respect to the blade A. Under these conditions, it is usual to design a Francis turbine runner so that the average fibre M of each blade is oriented along a straight line Δ making an angle α less than 90° with respect to the linear speed of advance U of its leading edge B.
There have been envisaged, in the Article “Why not make the turbines cavitation free” by Mr. Brekke (Proceedings of International Conference on Hydropower Vol. 3, 1997), different orientations for the leading edge of the blades of a Francis turbine runner.
However, particularly in the case of rehabilitation, the conditions of use of the turbine may be modified, particularly by reduction of the speed of rotation and/or increase of the pressure head, in which case the orientation of the leading edge of the blades is no longer compatible with the angle of incidence of the jet of water. In that case, eddies and/or phenomena of cavitation are created in the proximity of the pressure side and suction side surfaces of the blades, this reducing the efficiency of the hydraulic machine and promoting the phenomena of wear and tear.